1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a field sequential driving method and a liquid crystal display device using the same.
2. Description of the Related Art
Recently, personal computers and televisions have become lightweight and flat, and accordingly display devices are being required to be lightweight and flat. Thus, flat panel displays including a liquid crystal display (LCD) have been developed for use instead of a cathode ray tube (CRT).
An LCD device utilizes two substrates and a liquid crystal material having an anisotropic dielectric constant injected between the substrates, in which an electric field is applied to the liquid crystal material. The amount of light from an external light source transmitted through the substrates is controlled by intensity of the electric field to obtain a desired image signal.
Such an LCD is the most common type of flat panel displays, and especially, a thin film transistor (TFT)-LCD using a TFT as a switching element is most commonly used.
Each pixel in the TFT-LCD can be modeled as a capacitor having a liquid crystal as a dielectric material, that is a liquid crystal capacitor. An equivalent circuit diagram of such a pixel is shown in FIG. 1.
As shown in FIG. 1, each pixel in an LCD device includes a TFT 10 having a source electrode and a gate electrode respectively coupled to a data line Dm and a scan line Sn, a liquid capacitor Cl coupled between a drain electrode of the TFT 10 and a common voltage source Vcom, and a storage capacitor Cst coupled to the drain electrode of the TFT 10.
As can be seen in FIG. 1, the TFT 10 is turned on when a scan signal is applied to the scan line, and a data voltage Vd supplied to the data line Dm is applied to each pixel (not shown) through the TFT 10. Then, an electric field corresponding to a difference between a pixel voltage Vp and the common voltage Vcom is applied to a liquid crystal (equivalently shown as a liquid crystal capacitor Cl in FIG. 1), and light transmittance is determined by intensity of the electric field. Here, the pixel voltage Vp is maintained for one frame scan or one field, and the storage capacitor Cst is auxiliarily used to maintain the pixel voltage Vp applied to the pixel electrode.
In general, methods of displaying a color image on an LCD device can be classified into a color filter method and a field sequential driving method.
An LCD device employing the color filter method forms a color filter layer having 3 primary colors (red, green, and blue) on one of substrates, and a desired color is displayed by controlling the amount of light transmitted to the color filter. An LCD employing the color filter method transmits light emitted from a light source to red, green, and blue color filters, and the desired color can be expressed by controlling the amount of red, green, and blue lights transmitted through the red, green, and blue color filters and combining these lights.
Such an LCD device displaying colors using a single-light source and three color filter layers requires three times or more pixels compared to displaying monochrome to respectively correspond to red, green, and blue color areas. Accordingly, a sophisticated manufacturing technology is required to obtain a high resolution image.
Moreover, adding a separate color filter layer on the substrate of the LCD causes the manufacturing of the LCD to be complicated, and light transmittance of the color filter must be considered as well.
On the other hand, an LCD employing the field sequential driving method periodically and sequentially turns on/off independent red, green, and blue signals, and synchronously applies a corresponding color signal to the pixel in accordance with the turn on/off period to thereby obtain a full-colored image. In other words, the field sequential driving method uses persistence of vision to display a colored image by way of outputting the red, green, and blue (RGB) lights from RGB light sources (i.e., backlights) and time-dividing the RGB lights, and sequentially displaying the time-divided RGB lights on a pixel instead of dividing the pixel into three pixels for red, green, and blue colors.
The field sequential driving method can be classified into an analog driving method or a digital driving method.
The analog driving method predetermines a plurality of grayscale voltages corresponding to a total number of grayscales to be displayed, and selects a grayscale voltage corresponding to grayscale data from the plurality of grayscale voltages to drive a liquid crystal panel to thereby express grayscales using the amount of transmitted light corresponding to the grayscale voltage applied to the liquid crystal panel.
FIG. 2 illustrates a driving voltage and the amount of transmitted light according to an LCD panel employing a conventional analog driving method. As shown therein, the driving voltage represents a voltage applied to the liquid crystal, and the optical transmittance represents a ratio of the amount of light transmitted through the liquid crystal to the amount of incident light. In other words, the optical transmittance represents a ratio of the amount of light transmitted through the liquid crystal with respect to the degree of distortion of the liquid crystal.
Referring to FIG. 2, a driving voltage of V11 level is applied to the liquid crystal in an R-field period Tr for displaying a red color and the amount of light transmitted through the liquid crystal corresponds to the driving voltage. In a G-field period Tg for displaying a green color, a V12-level driving voltage is applied and a corresponding amount of light is transmitted through the liquid crystal. Further, in the B-field period Tb for displaying a blue color, a V13 level driving voltage is applied and a corresponding amount of light is transmitted through the liquid crystal. By combining the red, green, and blue lights respectively transmitted through the Tr, Tg, and Tb, a desired colored image can be displayed.
On the other hand, the digital driving method regulates driving voltages applied to the liquid crystal and controls a voltage application time to thereby express grayscales. According to the digital driving method, the grayscales are expressed by maintaining the regulated driving voltage and adjusting a timing or duration of the voltage application to control an accumulated amount of light transmitted through the liquid crystal.
FIG. 3 illustrates waveforms that explain a driving method of an LCD device employing a conventional digital driving method. Waveforms of a driving voltage in accordance with a predetermined number of bits of driving data and corresponding optical transmittance of a liquid crystal are illustrated.
As shown in FIG. 3, a 7-bit digital signal is provided as grayscale waveform data for each grayscale, and a corresponding grayscale waveform is applied to the liquid crystal. The optical transmittance of the liquid crystal is determined according to the applied grayscale waveform, thereby expressing the grayscales.
According to a conventional field sequential driving method, a measured value of a current grayscale (e.g., grayscale R) can be varied depending on a previous grayscale (e.g., grayscale B), and thus it is difficult to express accurate grayscale levels. In other words, a pixel voltage Vp supplied to a current liquid crystal is determined by grayscale voltages supplied to both a current field (e.g., field R) and a previous field (e.g., field B).
In particular, a value of the grayscale may be suddenly dropped because the field sequential method applies the grayscale voltage to one pixel in sequence of the field R, field G, and field B. Accordingly, previous grayscale data affects representing current grayscale data.
In an LCD device employing a general filter method, one pixel is divided into three sub pixels, and the grayscale data is applied to each sub pixel in accordance with the following sequence R1->R2, G1->G2, and B1->B2, whereas the field sequential driving method applies the grayscale data to one pixel in accordance with the following sequence R1->G1->B1->R2->G2->B2, thereby causing a sudden change of the grayscale. In a sequentially inputted image signal, the grayscale data applied to the R2 in sequence of R1 is generally not suddenly changed according to its characteristic, but R1, G1, and B1 data for expressing different colors may be suddenly changed. When grayscale data is suddenly changed in one pixel, the previous grayscale data greatly affects the currently displayed grayscale.